Various movement or position encoders for sensing linear, rotary or angular movement are currently available. These encoders are generally based on either optical systems, magnetic scales, inductive transducers, or capacitive transducers.
For optical encoders, a number of systems have been developed. One recent system that is compact and utilizes fewer parts than most previous systems is disclosed in U.S. Pat. No. 5,909,283, to Eselun. The system described in the '283 patent has a grating scale and readhead including a point source (laser diode in readhead), a Ronchi grating or holographic element, and a photodetector array. As described, the point source results in interference fringes having a spacing equal to that of the scale. The interference fringe light is transmitted through the Ronchi grating or holographic element to the photodetector array. The photodetector array is arranged to derive four channels of quadrature signals from the transmitted fringe light. However, the resulting encoder is still of a size that is relatively large or prohibitive for a number of applications.
One very compact system utilizing optical fibers as receivers is disclosed in U.S. Pat. No. 4,733,071, to Tokunaga. The system described in the '071 patent has a code member scale, and an optical sensor head comprising an optical fiber tip light emitter and two optical fiber tip receptors closely arranged along the code member measuring axis. The optical sensor head is rotated (yawed) to adjust phase difference between the two optical fiber tip receptors. However, the accuracy of the resulting encoder is relatively crude. Another optical encoder utilizing optical fibers as receivers is disclosed in U.S. Pat. No. 4,291,976 to McMahon. The system described in the '976 patent includes optical fibers with striped ends positioned opposite relatively movable coded channel patterns, permitting the detection of motions that are comparable to the stripe width rather than the fiber diameter. However, the accuracy and resolution of the resulting encoder is still relatively crude compared to that provided by an interferometric-type encoder, and compared to that needed for a number of applications.
In various applications, it is desirable to use a so-called interferometric-type optical encoder, in order to achieve high resolution and accuracy. However, known interferometric-type optical encoders are not sufficiently compact for a number of applications, or they employ electronic detectors that limit the possible output frequency and motion speeds, or both.